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Tunneling Times and Superluminality: a Tutorial
Experiments have shown that individual photons penetrate an optical tunnel
barrier with an effective group velocity considerably greater than the vacuum
speed of light. The experiments were conducted with a two-photon parametric
down-conversion light source, which produced correlated, but random, emissions
of photon pairs. The two photons of a given pair were emitted in slightly
different directions so that one photon passed through the tunnel barrier,
while the other photon passed through the vacuum. The time delay for the
tunneling photon relative to its twin was measured by adjusting the path length
difference between the two photons in a Hong-Ou-Mandel interferometer, in order
to achieve coincidence detection. We found that the photon transit time through
the barrier was smaller than the twin photon's transit time through an equal
distance in vacuum, indicating that the process of tunneling in quantum
mechanics is superluminal. Various conflicting theories of tunneling times are
compared with experiment.Comment: 13 pages, Latex, 4 Figure
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